The Vitamin D metabolites known as 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3 (collectively referred to as “25-hydroxyvitamin D”) are fat-soluble steroid prohormones to Vitamin D hormones that contribute to the maintenance of normal levels of calcium and phosphorus in the bloodstream. The prohormone 25-hydroxyvitamin D2 is produced from Vitamin D2 (ergocalciferol) and 25-hydroxyvitamin D3 is produced from Vitamin D3 (cholecalciferol) primarily by one or more enzymes located in the liver. The two prohormones also can be produced outside of the liver from Vitamin D2 and Vitamin D3 (collectively referred to as “Vitamin D”) in certain cells, such as enterocytes, which contain enzymes identical or similar to those found in the liver.
The prohormones are further metabolized in the kidneys into potent hormones. The prohormone 25-hydroxyvitamin D2 is metabolized into a hormone known as 1α,25-dihydroxyvitamin D3; likewise, 25-hydroxyvitamin D3 is metabolized into 1α,25-dihydroxyvitamin D3 (calcitriol). Production of these hormones from the prohormones also can occur outside of the kidney in cells which contain the required enzyme(s).
The Vitamin D hormones have essential roles in human health which are mediated by intracellular Vitamin D receptors (VDR). In particular, the Vitamin D hormones regulate blood calcium levels by controlling the absorption of dietary calcium by the small intestine and the reabsorption of calcium by the kidneys. Excessive hormone levels, whether transient or prolonged, can lead to abnormally elevated urine calcium (hypercalciuria), blood calcium (hypercalcemia) and blood phosphorus (hyperphosphatemia). The Vitamin D hormones also participate in the regulation of cellular differentiation and growth, PTH secretion by the parathyroid glands, and normal bone formation and metabolism. Further, Vitamin D hormones are required for the normal functioning of the musculoskeletal, immune and renin-angiotensin systems. Numerous other roles for Vitamin D hormones are being postulated and elucidated, based on the documented presence of intracellular VDR in nearly every human tissue.
The actions of Vitamin D hormones on specific tissues depend on the degree to which they bind to (or occupy) the intracellular VDR in those tissues. The prohormones 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3 have essentially identical affinities for the VDR which are estimated to be at least 100-fold lower than those of the Vitamin D hormones. As a consequence, physiological concentrations of 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3 have little, if any, biological actions without prior metabolism to Vitamin D hormones. However, supraphysiologic levels of 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3, in the range of 10 to 1,000 fold higher than normal, can sufficiently occupy the VDR to exert actions like the Vitamin D hormones.
Surges in blood or intracellular prohormone concentrations can promote excessive extrarenal hormone production, leading to local adverse effects on calcium and phosphorus metabolism. They also can inhibit hepatic prohormone production from Vitamin D, and promote catabolism of both Vitamin D and 25-hydroxyvitamin D in the kidney and/or other tissues. Blood levels of both the prohormones and the Vitamin D hormones are normally constant through the day, given a sustained, adequate supply of Vitamin D from sunlight exposure or an unsupplemented diet. Blood levels of 25-hydroxyvitamin D, however, can increase markedly after administration of currently available Vitamin D supplements, especially at doses which greatly exceed the minimum amounts required to prevent Vitamin D deficiency rickets or osteomalacia. Prohormone blood levels can also increase markedly after rapid intravenous administration of 25-hydroxyvitamin D2 and/or 25-hydroxyvitamin D3.
Production of 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3 declines when Vitamin D is in short supply, as in conditions such as Vitamin D insufficiency or Vitamin D deficiency (alternatively, hypovitaminosis D). Low production of 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3 leads to low blood levels of 25-hydroxyvitamin D. Inadequate Vitamin D supply often develops in individuals who are infrequently exposed to sunlight without protective sunscreens, have chronically inadequate intakes of Vitamin D, or suffer from conditions that reduce the intestinal absorption of fat soluble vitamins (such as Vitamin D). It has recently been reported that most individuals living in northern latitudes have inadequate Vitamin D supply. Left untreated, inadequate Vitamin D supply can cause serious bone disorders, including rickets and osteomalacia, and may contribute to the development of many other disorders including osteoporosis, non-traumatic fractures of the spine and hip, obesity, diabetes, muscle weakness, immune deficiencies, hypertension, psoriasis, and various cancers.
The Institute of Medicine (IOM) of the National Academy of Sciences has concluded that an Adequate Intake (AI) of Vitamin D for a healthy individual ranges from 200 to 600 IU per day, depending on the individual's age and sex [Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, Dietary reference intakes: calcium, phosphorus, magnesium, vitamin D, and fluoride. Washington, D.C.: National Academy Press (1997)], incorporated by reference. The AI for Vitamin D was defined primarily on the basis of a serum 25-hydroxyvitamin D level sufficient to prevent Vitamin D deficiency rickets or osteomalacia (or ≧11 ng/mL). The IOM also established a Tolerable Upper Intake Level (UL) for Vitamin D of 2,000 IU per day, based on evidence that higher doses are associated with an increased risk of hypercalciuria, hypercalcemia and related sequelae, including cardiac arrhythmias, seizures, and generalized vascular and other soft-tissue calcification.
Currently available oral Vitamin D supplements are far from ideal for achieving and maintaining optimal blood 25-hydroxyvitamin D levels. These preparations typically contain 400 IU to 5,000 IU of Vitamin D3 or 50,000 IU of Vitamin D2 and are formulated for quick or immediate release in the gastrointestinal tract. When administered at chronically high doses, as is often required for Vitamin D repletion, these products have significant and, often, severe limitations which are summarized below.
High doses of immediate release Vitamin D supplements produce marked surges in blood Vitamin D levels, thereby promoting: (a) storage of Vitamin D in adipose tissue, which is undesirable because stored Vitamin D is less available for later conversion to 25-hydroxyvitamin D; (b) catabolism of Vitamin D to metabolites which are less or no longer useful for boosting blood 25-hydroxyvitamin D levels, via 24- and/or 26-hydroxylation; and, (c) excessive intracellular 24- or 25-hydroxylation of Vitamin D, which leads to increased risk of hypercalciuria, hypercalcemia and hyperphosphatemia via mass-action binding to the VDR.
High doses of immediate release Vitamin D supplements also produce surges or spikes in blood and intracellular 25-hydroxyvitamin D levels, thereby promoting: (a) transiently excessive renal and extrarenal production of Vitamin D hormones, and leading to local aberrations in calcium and phosphorus homeostasis and increased risk of hypercalciuria, hypercalcemia and hyperphosphatemia; (b) catabolism of both Vitamin D and 25-hydroxyvitamin D by 24- and/or 26-hydroxylation in the kidney and other tissues; (c) down-regulation of hepatic production of Vitamin D prohormones, unnecessarily impeding the efficient repletion of Vitamin D insufficiency or deficiency; and, (d) local aberrations in calcium and phosphorus homeostasis mediated by direct binding to VDR.
Furthermore, high doses of immediate release Vitamin D supplements produce supraphysiologic, even pharmacological, concentrations of Vitamin D, e.g., in the lumen of the duodenum, promoting: (a) 25-hydroxylation in the enterocytes and local stimulation of intestinal absorption of calcium and phosphorus, leading to increased risk of hypercalciuria, hypercalcemia and hyperphosphatemia; and (b) catabolism of Vitamin D by 24- and 26-hydroxylation in the local enterocytes, causing decreased systemic bioavailability.
Vitamin D supplementation above the UL is frequently needed in certain individuals; however, currently available oral Vitamin D supplements are not well suited for maintaining blood 25-hydroxyvitamin D levels at optimal levels given the problems of administering high doses of immediate release Vitamin D compounds.
Administration of 25-hydroxyvitamin D3 in an immediate release oral formulation has been tried as an alternative method of Vitamin D supplementation. This approach, which has been subsequently abandoned, caused problems as do the currently used Vitamin D supplements. Specifically, it produced surges or spikes in blood and intracellular 25-hydroxyvitamin D levels, thereby promoting (a) competitive displacement of Vitamin D hormones from the serum Vitamin D Binding Protein (DBP) and excessive delivery of the displaced hormones to tissues containing VDR, and (b) transiently excessive renal and extrarenal production of Vitamin D hormones, which together led to local aberrations in calcium and phosphorus metabolism. In addition, these surges in blood 25-hydroxyvitamin D levels promoted catabolism of both Vitamin D and 25-hydroxyvitamin D by 24- and/or 26-hydroxylation in the kidney and other tissues, down-regulation of hepatic production of Vitamin D prohormones, unnecessarily impeding the efficient repletion of Vitamin D insufficiency or deficiency, and, additional local aberrations in calcium and phosphorus homeostasis mediated by direct binding to VDR. Importantly, immediate release 25-hydroxyvitamin D3 promoted its intestinal absorption via a mechanism substantially involving transport to the liver in chylomicrons, rather than bound to the serum DBP. Delivery of 25-hydroxyvitamin D to the liver via chylomicrons significantly increased the likelihood of its catabolism.
Clearly, an alternative approach to Vitamin D supplementation is needed given the problems encountered with both currently available oral Vitamin D supplements, and with previously used oral 25-hydroxyvitamin D3.